The present invention relates generally to coal-fired power plants and in particular to an assembly for ash separation from flue gas.
It is well-known that coal and other solid fuels generate significant quantities of ash as a byproduct of combustion. The ash typically collects at the bottom of the furnace, but significant quantities of ash becomes entrained in the flue gas stream exiting the furnace. Hoppers for the collection of ash from the flue gas stream are typically positioned downstream from the furnace in order to accumulate the ash entrained in the flue gas stream.
The carry over of ash particulate into equipment located downstream of the furnace, is a severe problem. The catalytic converter reduces the NOx levels in flue gas. The catalytic converter typically includes packings of narrow channels having catalyzer surfaces. The ash in the flue gas accumulates on the catalyzer surfaces which results in clogging of the catalyzer channels and the channel walls increasingly losing their catalytic effect.
In addition to a reduction of the catalytic effect, the ash accumulation on the catalyzer channel wall results in pressure loss due to the reduction of the flow cross-section. The ash particulate lodges into the surfaces of the downstream equipment and results in plugging of air passages and increased pressure differentials. It often becomes necessary to shut down the furnace and clean, repair or replace the catalytic converter. The replacement of the catalytic converters is extremely expensive, and in particular on the one hand due to the unavoidable shutdown of the power station, and on the other hand due to the procurement and assembly costs of the catalytic converters.
Conventional power plants employ an ash hopper upstream from the catalytic converter to collect ash particulate from the flue gas stream. Screens can be positioned across the flue gas stream at the outlet duct of the ash hopper in order to increase ash removal. The screens can cover the entire duct, or cover only a portion of the duct and be staggered with additional partial screens. The screens are typically positioned vertically across the horizontal section of the outlet duct downstream of the hopper, or can be tipped away from the hopper at their upper edges. In either the vertical arrangement of the screens or where the screens are tipped away from the hopper, the pressure of flue gas stream tends to hold the ash in position on the screen once the ash has impacted the screen surface. The holding of the ash against the screen results in plugging of the screen and decreased flow of the flue gas stream. The plugging of the screens also increases power plant operating cost.